As thin film transistors used in display devices such as liquid crystal displays and various control circuits, there are thin film transistors in which an oxide semiconductor is used as a semiconductor layer.
Such thin film transistors include, for example, an oxide semiconductor layer, source electrode, and drain electrode above an insulating substrate. In an etching process to form the source electrode and the drain electrode, the oxide semiconductor layer may be damaged and it may cause a shift in a threshold value voltage and a deterioration in a TFT performance. Therefore, there is a proposed technique to maintain a thin conductive layer between the source electrode and the drain electrode while performing etching of the source electrode and the drain electrode and to cause a chemical action in the semiconductor layer into a semiconductor or an insulating material.